1. Field of the Invention
This invention relates to flow control apparatus, and in particular, to a flow control device having an unbalance pressure actuator located in a noise and vibration reduction and control arrangement.
2. Description of the Prior Art
In general, a steam turbine power plant, whether fossil fuel or nuclear, comprises a series-connected arrangement of at least a steam generator element, a turbine element, and a condenser element. It is, of course, well known that the turbine element converts the energy contained within high pressure and high temperature motive steam to rotational mechanical energy which is transmitted by a common shaft to an associated electrical generator element. Suitable flow control apparatus, including at least one control valve, is connected within the conduit arrangement disposed between the steam generator and the turbine element. The control valve, in addition to providing steam interdiction capability in the event of a system malfunction, has as its function the regulation of the steam flow to the turbine element from the steam generator element.
The steam conducted into the turbine element enters the turbine through what is known in the art as an admission arc, the arc being located in the turbine immediately preceding the first row of stationary blade elements mounted therein. It is known in the art to provide a turbine element having disposed therein a plurality of admission arcs, each arc being said to be a partial admission arc. If a turbine is provided with such a plurality of partial admission arcs, there is disposed in the steam line between each admission arc and the steam generator element a suitable flow control device, such as a control valve.
When low load conditions are imposed upon the entire generating facility, such as during off-peak periods, it is common practice to reduce the mass flow of motive fluid passing from the steam generator to the turbine element. In a single admission arc machine, the reduction in mass flow is accomplished by modulating the position of a valve plug within the control valve relative to its associated valve seat. Modulation of the valve plus reduces the pressure of the motive fluid entering the turbine, and, since the admission area of the single admission arc turbine remains constant, modulation of the valve thereby reduces the flow rate entering the turbine. Of course, if a partial admission arc machine is utilized, reduction of mass flow rate into the turbine can be accomplished by sequentially reducing the area available to the flow, that is, by utilizing only a predetermined number of the plurality of admission arcs available. Furthermore, the pressure of the motive fluid entering each of the admission arcs is regulated by modulating the location of the control valve plug governing the flow of motive fluid into each admission arc.
It has been found however, that during low load conditions, when the ratio of the valve inlet pressure relative to the turbine inlet pressure is high, excessive noise and vibration levels occur within the control valve. Such excessive noise and vibration levels are mainly due to vortex excitation forces imposed upon the bottom of the control valve plug and by the direct collision of fluid flow stream lines beneath the control valve plug.
With the valve plug in a modulating position relative to its associated valve seat, the motive fluid flow through the valve must, of necessity, pass through the channel defined between the plug and the seat. For a given "lift" or clearance between the plug and the seat, the high pressure ratios between valve inlet and turbine inlet which occur during low load conditions and the associated downstream flow field at these pressure ratios cause the high velocity fluid which enters the downstream piping beneath the control valve plug to separate from the boundaries of the downstream piping and to collide underneath the control valve plug. The collision of the fluid stream lines creates a closed vortex cavity underneath the valve plug, the cavity generally being asymmetric relative to an axis extending through the downstream conduit. Such a closed vortex cavity causes the motive fluid confined therein to pound upon the bottom of the control valve plug. This pounding by the turbulent fluid trapped within the closed vortex cavity causes vibration and pulsating forces to be imposed upon the control valve plug. Further, the collision of the stream lines beneath the control valve plug generates excessive noise levels.
When the pressure ratio between the valve inlet and the turbine inlet is at a lower value, or when the clearance or lift between the valve plug and valve seat is increased, the fluid flow stream lines passing through the channel between the valve plug and the valve seat generally flow along the boundaries of the piping downstream of the valve. As a result, it is established that an increase in the static pressure occurs within the conduit approximately five pipe diameters downstream of the point of entry of the fluid that is, downstream of the channel between the valve plug and the valve seat. The static pressure downstream is greater than the static pressure immediately beneath the valve plug. This difference in static pressure causes a recirculating back flow of fluid moving in an upstream direction toward the control valve plug in the central portion of the flow. This back flow is advantageous in that it avoids the above-mentioned pounding and colliding effects. Although the interaction of the recirculating back flow and the high velocity inlet steam causes some shear noise and some vibration for the lower inlet pressure ratios or for higher lift, these effects are not deleterious.
However, as the valve-inlet-to-turbine inlet pressure ratio increases, the direct collision of the fluid stream lines beneath the valve plug prevent the back flow of recirculating fluid from occurring. Thus, at the higher inlet pressure ratios the velocity head of the inlet fluid is not dissipated by the viscous interaction between it and the recirculated back flow.
For the situation of steam line collision it may be said, in general, that the pressure at the inlet of the valve is always greater than the pressure approximately five diameters downstream of the plug. If this were not the case there could be no flow through the valve. It may also be generally said that the pressure downstream of the valve is greater than the pressure along the walls of the outlet piping immediately adjacent the valve seat.
In the copending application of D. Q. Hoover, Ser. No. 563,422, filed Mar. 28, 1975 filed concurrently herewith, the fact that the downstream pressure is greater than the pressure along the sidewalls of the piping is utilized to cause separation between the colliding stream lines. Suitable piping means are provided external to the valve to conduct motive fluid from the higher pressure region downstream of the valve plug into and through the control valve. Thus, the back flow which naturally occurs in the downstream conduit but which was cut off by the collision of the influent stream lines is provided an alternate route through the external piping. The fluid conducted from the higher pressure region downstream of the valve plug is introduced through openings in the plug directly into the region immediately beneath the valve plug. Since the pressure of the downstream fluid is greater than the pressure along the side walls of the outlet conduit, the inlet main streams are forced apart by the pressurized fluid, thus opening the asymmetric closed vortex and reducing vibration excitation of the plug. At the same time, shear noise is reduced by preventing collision of the inlet main streams.
However, there are situations when the pressure beneath the valve plug within the vortex cavity exceeds the pressure downstream in the conduit. In this case, a counter flow through the valve plug to the downstream region will occur. If this counter flow exists, the flow of motive fluid to the turbine will be interdicted and a possible turbine overspeed condition, with the attendant possibility of turbine damage, can occur. In order to prevent the deleterious effects of a counter flow through the control valve, an unbalanced pressure actuator valve embodying the teachings of this invention is provided within the external conduit line.